Even as the Kepler space telescope was making one amazing planet discovery after another after another over the past year or so, it was dawning on astronomers working on the mission that their ultimate goal would be tougher to achieve than they'd hoped. Kepler, which was launched in 2009, was designed to find Earth-size planets in the life-friendly habitable zones of sunlike stars. At the minimum, the scientists thought they'd need three years to bag these elusive worlds, but that turned out to be too optimistic. They'd need six or seven, and the mission would run out of money long before then.

Now the word has finally come down, and the Kepler scientists are experiencing an enormous sense of relief  and in some cases, more. "The decision to extend the Kepler mission," says Geoff Marcy, a Berkeley astrophysicist, Kepler team member and, even before Kepler came along, the world's greatest planet hunter, "brought tears to my eyes."

They're all crucial to understanding the universe, especially when several of them are trained on a single cosmic object, which just happened in an investigation into the death of two galaxies. In Kepler's case, however, the scientific value is a lot more obvious to the average taxpayer. How galaxies die is one thing, but the question of how many earthlike worlds are out there in the Milky Way is something everyone can relate to. Astronomers have suspected for decades that there must be plenty of them; that's the reasoning behind the half-century-old Search for Extraterrestrial Intelligence, or SETI.

Kepler won't discover aliens, but it will give astronomers a better idea of how hard they'll have to look in order to find them. The probe is peering around the clock at more than 150,000 stars in a small patch of sky between the constellations Cygnus and Lyra, waiting for them to blink as an otherwise invisible planet passes in front of them. So far, Kepler has officially found what appear to be 123 Earth-size planets, along with more than 2,000 others, and there are almost certainly many more still in the pipeline.

The catch is that these alien Earths are all too close to their stars, and thus too hot to support life. To find an Earth-size world around a sunlike star, you have to wait a year for each blink, and that has to repeat three times for the Kepler team to consider it a legitimate candidate. The original three years of life NASA gave the mission should have been enough time to find such planets  but by last summer, the Kepler scientists realized that sunlike stars are "noisier" than they expected, with more sunspots and flares than our own sun has. As a result, says Marcy: "So far, Kepler has found none, zero, goose egg."

But the noise doesn't make it impossible to find Earths; it just takes more time  something Kepler now has. "The news this week has lightened the mood considerably," says Kepler scientist Natalie Batalha, of the NASA Ames Research Center and San Diego State University, in what's clearly an understatement. "We're breathing easier. More importantly, we're rolling up our sleeves and getting back to work. Time to get busy!"

Marcy, the venerable planet hunter, waxes much more poetic on the topic. "Like a cosmic cartographer," he says, "Kepler will give us the exact coordinates to prospect for earthlike planets in the Milky Way. Only the passage of centuries will tell us if these new worlds offer safe harbor, and perhaps even treasures, for our fledgling species, as we take our first tentative steps beyond our birth planet."